Large pumps are commonly used for mining and oilfield applications, such as, for example, hydraulic fracturing. During hydraulic fracturing, fracturing fluid (i.e., cement, mud, frac sand and other material) is pumped at high pressures into a wellbore to cause the producing formation to fracture. One commonly used pump in hydraulic fracturing is a high pressure reciprocating pump, like the SPM® QWS 3500 frac pump, manufactured by S.P.M. Flow Control, Inc. of Fort Worth, Tex. In operation, the fracturing fluid is caused to flow into and out of a pump housing having a fluid chamber as a consequence of the reciprocation of a piston-like plunger respectively moving away from and toward the fluid chamber. As the plunger moves away from the fluid chamber, the pressure inside the chamber decreases, creating a differential pressure across an inlet valve, drawing the fracturing fluid through the inlet valve into the chamber. When the plunger changes direction and begins to move towards the fluid chamber, the pressure inside the chamber substantially increases until the differential pressure across an outlet valve causes the outlet valve to open, enabling the highly pressurized fracturing fluid to discharge through the outlet valve into the wellbore.
A typical reciprocating pump includes multiple lubrication systems: a fluid end lubrication system that lubricates and cools the bearing surfaces of a fluid end, and a power end lubrication system that lubricates and cools the rolling and sliding of, for example bearing, surfaces of a power end. In the power end, it can be beneficial to supply some rolling and sliding surfaces with a higher pressure of lubrication fluid than other rolling and sliding surfaces. In present systems, however, the rolling and sliding surfaces of the power end are lubricated by the same lubrication circuit and thus, are generally lubricated at the same lubrication fluid pressure.
In operation, the pressure of the lubrication fluid received by a particular surface depends on the flow of lubrication fluid from the lube pump and the resistance to the flow created by the outlets in the lubrication circulating system. Because some components, such as roller bearings and gears, have lubrication fluid (i.e., oil) flowing out at approximately atmospheric pressure, the single circuit lubrication system oftentimes fails to provide sufficient lubrication fluid pressure and flow to ensure that all parts, especially sliding surfaces, which can require a higher lubrication fluid pressure, are properly lubricated. In order to ensure adequate lubrication of the power end, the required lubrication pressure and flow rate to all of the rolling and sliding surfaces is increased; however, such increases create inefficiencies in the power end lubrication system and thus, inefficiencies in the operation of the reciprocating pump.